1. Introduction

Fiber lasers have been Q-switched using traditional bulk Q-switches and fiber-pigtailed ones. Sophisticated alignment techniques were necessary for reducing the coupling losses between the bulk Q-switches and the fiber cores. High-power, multimode pump diodes, double-cladding fibers and pump-signal combiners were often employed to increase pump efficiency and build up large laser gains that compensated for the coupling losses. All-fiber Q-switched lasers with fiber-pigtailed Q-switches are of great interest because of their alignment-free characteristics and low cavity loss that is essential for efficient Q-switching performance. The fiber-pigtail Q-switches can be active devices as piezoelectric actuators [1], acousto-optical modulators [2–4] and magnetostrictive transducers [5], or passive ones such as semiconductor saturable-absorber mirrors (SESAM) [6,7] and saturable-absorber fibers [8–12]. Passive Q-switching with a saturable absorber fiber is the simplest and most economical approach for producing high-power laser pulses. In addition, a solid-state saturable absorber Q-switch (SAQS) fiber has a high damage threshold and can hold an enormous laser gain for energy release into one Q-switched laser pulse. In spite of these advantages, the SAQS fibers are seldom considered in the literature. One of the criteria for a SAQS laser is that the absorption cross section, σ_a, of the SAQS should be larger that the emission cross section, σ_e, of the gain medium. Furthermore, to achieve sequentially Q-switched pulses using a continuous-wave (CW) pump source, the relaxation lifetime of the SAQS, τ_a2, should be shorter than that of the gain medium, τ_g2. There has been a breakthrough in the last four years in finding SAQS fibers for ytterbium fiber lasers. Fibers doped with bismuth [8] and rare earth ions such as samarium [9], thulium [10] and holmium [11] have been demonstrated as efficient SAQS materials for ytterbium fiber lasers within the wavelength range from 1050 nm to 1125 nm where the σ_e of Yb is relatively small. Moreover, a Ho-doped fiber was reported as a SAQS fiber for use in a Tm fiber laser operating at 2-2.1 μm [12].

Erbium fiber lasers emitting at the so-called eye-safe wavelength of 1.5-1.6 μm are the preferred laser devices for light detection and ranging (LIDAR) and other applications that require Q-switched operation. Few bulk crystals such as U:CaF₂ [13] Co:ZnS and Co:ZnSe [14] have been reported to be efficient SAQS materials for Er:glass lasers. Nevertheless, there has been no fiber-type SAQS material demonstrated for erbium fiber lasers except for erbium fiber itself. Recently, we demonstrated self-Q-switched, all-fiber erbium lasers in a ring scheme [15] and a standing-wave resonator [16], where erbium fibers served as both 3-level lasers and 2-level saturable absorbers. Since the cross sections, σ_a and σ_e, of erbium are comparable, techniques of photon density enhancement in the SAQS fibers, such as double-passing routes and mismatch of mode field areas (MFA), were developed to activate SAQSing functions. Due to lifetime matching, τ_a2 = τ_g2, in a self-Q-switched laser, a saturable-amplifier pump switch (SAPS) was introduced for acquiring sequentially Q-switched pulses [16]. The function of the SAPS was to passively switch the pump intensity and delay the recovery of the gain population after Q-switched pulses. Although a passively Q-switched, all-fiber laser was demonstrated at low repetition rates, the peak pulse power decreased quickly (i.e. less pulse energy and larger pulse width) with an increasing pump power. The peak pulse power was 100 W at the lowest repetition rate of 250 Hz and decreased to 20 W at 1 kHz (i.e. ~10/τ₂). The low-rate Q-switching was primarily due to the long relaxation lifetime of erbium (10 ms). Techniques shortening the relaxation lifetime of the erbium SAQS fiber could solve this inherent drawback and broaden the applications of the device.

In this paper, we demonstrate a self-Q-switched, all-fiber, erbium laser that emits at 1530 nm using an auxiliary 1570 nm erbium laser applied to the SAQS erbium fiber. Both wavelengths, 1530 and 1570 nm, are in the same transition band of erbium. The dopant Er³⁺ has a broad band of energy transition between 4I_13/2 and 4I_15/2, which corresponds to an emission and absorption wavelength range from 1.48 to 1.6 μm. According to the specifications of the SAQS erbium fiber provided by the manufacturer nLight, the emission and absorption cross sections at 1530 nm, σ_e,1530 and σ_a,1530 are about the same: 6 × 10⁻²¹ cm². The cross sections, σ_e,1570 and σ_a,1570, are 2.6 × 10⁻²¹ cm² and 1.3 × 10⁻²¹ cm². These values might differ slightly for different products and manufacturers. The effective absorption population, defined as N_a,λ = [ N_a1-(σ_e,λ/σ_a,λ)N_a2], is wavelength-dependent, and the cross-section ratio, σ_a,λ /σ_e,λ, is the population ratio of N_a2 to N_a1 in the state of full absorption saturation, that is to say, N_a,λ = 0. When the SAQS is fully saturated by a large 1570-nm laser, I_a,1570, we have N_a,1570 = 0, N_a2 = N_aT/3 and N_a1 = 2N_aT/3 that indicates N_a,1530 = N_aT/3, where N_aT is the total erbium dopants of the SAQS fiber. Therefore, the initial value of N_a,1530, called N_ai, for Q-switching can be tuned between N_aT and N_aT/3 with an I_a,1570 applied on the SAQS fiber. More importantly, an I_a,1570 can shorten the relaxation lifetime of N_a,1530. When the SAQS fiber is bleached by a Q-switched pulse at 1530nm, N_a,1530 is zero and N_a,1570 is -N_aT/2 (i.e. gain in the SAQS for I_a,1570). This instant status will soon be modified by the I_a,1570 back to the initial condition, N_a,1530 = N_ai, for next Q-switching. To simplify the discussion on the effect of I_a,1570 on the erbium lifetime, we assumed uniform distribution of N_a2, N_a1 and I_a,1570 in the SAQS and derived the effective lifetime of N_a,1530 as:

2. Experiments

Figure 1 shows the schematic design of a self-Q-switched, erbium, all-fiber laser, pumped with a continuous-wave (CW) 980-nm laser diode (LD). The Q-switched laser was stabilized, and the pulse repetition rate was tunable using a 1570-nm laser that was also an erbium fiber laser pumped with a 980-nm LD (not shown). Therefore, the laser system could be simplified using only one high-power pump LD and a pump splitter with a properly designed ratio. The gain medium was a 210-cm erbium-doped fiber with a core diameter of 14 μm and an absorption loss of 19 dB⋅m⁻¹ at 1530 nm, manufactured by the company Coractive. The SAQS was a 20-cm Er fiber, made by the manufacturer nLight, with a relatively smaller core diameter of 4 μm and an absorption loss of 110 dB⋅m⁻¹ at 1530 nm. The mismatch of the mode field areas (MFA) between the gain and the SAQS resulted in high photon density and fast absorption saturation in the SAQS fiber, thereby giving rise to Q-switching action. The numerical aperture (NA) number and the mode field diameter of the SAQS were 0.2 and 6.5 μm, indicating a confinement factor Γ of about 0.53. Thus, the saturation power, I_s,1570, was determined by Eq. (1) to be 1.2 mW. The 980/1530 nm WDM inside the resonator was used to protect the SAQS from the pump power. Similarly, a 1530/1570 nm WDM was employed to prevent the gain fiber from being stimulated by the 1570-nm laser. All components were core-fusion spliced. The length of the resonator was about 4 meters.

 

Fig. 1 Schematic diagram of a self-Q-switched, all-fiber erbium laser at 1530 nm with a tunable repetition rate using an auxiliary 1570 nm laser.

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The Q-switching performances related to the pulse repetition rate, R_pr, with I_a,1570 = 0 and I_a,1570 = 10 mW are compared and shown in Fig. 2(a) and (b) . Both the cases with and without I_a,1570 were stable when R_pr was within 0.9-10 kHz and unsteady when R_pr was below 0.9 kHz. Instead of stabilizing the laser at low R_pr (<1 kHz) as the function of a saturable-amplifier pump switch (SAPS) [16], the tuning source, I_a,1570, primarily improved the Q-switching efficiency at high R_pr by affecting the relaxation lifetime. Without the I_a,1570, a pulse had a full-width-at-half-magnitude (FWHM) of about 0.9 μs and an energy of 1.1 μJ at a repetition rate, R_pr, of 0.9 kHz near the laser threshold. The 0.9 kHz value of R_pr indicated a 1.1-ms recovery time (i.e. τ_a2/9) for N_a,1530 after being bleached by a Q-switched pulse. Assuming the SAQS was fully bleached by each pulse, it can be calculated that N_a,1530 switched between 0 to about 0.1N_aT. A higher pump power would give less time for N_a,1530 recovery, leading to a higher repetition rate and smaller pulse energy. The pulsing output was still observed with a pump power larger than 100 mW where the pulse had a pulse width of microseconds and a very low pulse energy. Such low-efficiency Q-switching is referred to as “Q-fluctuation” and can only hold a small amount of energy in the gain medium and will have low extraction efficiency of the gain population.

 

Fig. 2 The Q-switching performances with I_a,1570 = 0 and I_a,1570 = 10 mW. (a) Pulse energy and pump power related to pulse repetition rate, (b) pulse peak power and pulse FWHM related to pulse repetition rate.

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When the 1570-nm laser was turned on, the Q-switching performance was much improved. By comparing the two cases (i.e. with and without I_a,1570) at the same R_pr in Fig. 2(a), the improved pulse energy with the I_a,1570 demonstrates faster and more extensive recovery of N_a,1530. Due to this increased N_a,1530 recovery, a higher pump power was required with the I_a,1570. Furthermore, the improved recovery of N_a,1530 denoted a higher hold-off ratio of the gain population in the gain fiber, in turn leading to a better Q-switching performance with a shorter pulse width and higher peak power as clearly demonstrated in Fig. 2(b).

The relation between the pulsing characteristics and the I_a,1570 with a constant pump power of 60 mW is shown in Fig. 3 . As indicated in Eq. (1), a more intense I_a,1570 will contribute a shorter τ_a2^’. Due to the increased N_a,1530 recovery that occurs with a shorter τ_a2^’, more time for pumping was needed to compensate for the increased N_a,1530, which resulted in a decreasing R_pr. Consequently, better pulsing characteristics with an increasing I_a,1570, as higher pulse energy, shorter pulse width and stronger peak power, were achieved. It is interesting to note that the decrease in R_pr, and the increasing of pulse energy with the I_a,1570 nearly stopped when the I_a,1570 approached 10 mW. Such a steady state implied that the I_a,1570 was sufficient to provide for a nearly full recovery of N_a,1530 back to the initial value of N_ai for each subsequent Q-switched pulse. When the I_a,1570 was sufficient, the Q-switching efficiency depended upon the designed N_ai and the fundamental physics of optimization.

 

Fig. 3 Q-switching performance related to the I_a,1570 using a constant pump power of 60 mW.

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We further improved the Q-switching performance by doubling the length of a SAQS fiber. The SAQS had an absorption loss of 44 dB at 1530 nm that was even larger than that of the gain fiber. Thus, the laser could not reach the threshold by pumping alone without the assistant of the I_a,1570. Stable, sequential, Q-switched pulses were achieved using a 10-mW I_a,1570, as shown in Fig. 4 . The pulse had a stable shape, a FWHM of about 40 ns, and a peak power of larger than 100 W along the pump range from 75 to 200 mW. The maximum pulse energy of 6 μJ and peak power of 165 W was achieved at the lowest R_pr of 0.1 kHz. The R_pr was proportional to the pump power and limited by the maximum 980-nm LD output. The high Q-switching efficiency was attributed to the high hold-off ratio of the gain population by the large N_ai, which, in turn, lead to a high extraction efficiency of the pumped gain. By recording the readings on the oscilloscope, statistics showed that the deviation of the repetition rate, ΔR_rp/R_rp was about 7% - 8% and the deviation of the peak power was about 8%. The stability showed no strong relation with R_rp, although it seemed that the stability was a little better (smaller ΔR_rp/R_rp) when R_rp was higher. Since more pumping time was needed for the large gain population, the Q-switching was stabilized into a low-R_pr range from 0.1 to 1.5 kHz. Efficient Q-switching at higher R_pr should be achievable using a more intense pump LD. The results demonstrated here were better and more stable than what could be achieved using a saturable-amplifier pump switch [16].

 

Fig. 4 (a) Q-switching performance using a 10-mW I_a,1570 and a SAQS erbium fiber with an absorption loss of 44 dB at 1530nm. (b) Sequential Q-switched pulses at 1.5 kHz captured on an oscilloscope, and (c) the corresponding pulse with a peak power of 105 W.

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3.Conclusion

We have demonstrated a self-Q-switched, all-fiber erbium laser emitting at 1530 nm through the use of an auxiliary laser at 1570 nm, I_a,1570, that allowed tunable and optimizable Q-switching performance. The I_a,1570 was applied to a SAQS erbium fiber to shorten the relaxation lifetime. The wavelengths of 1530 and 1570 nm are in the same band of energy transition between 4I_13/2 and 4I_15/2. A SAQS fiber with a 22-dB absorption loss was employed for demonstrating the effect of lifetime shortening, and the improvement on Q-switching at the repetition rate from 0.9 to 10 kHz. By doubling the length of the SAQS fiber and applying a 10-mW I_a,1570, sequential pulses with pulse energy of 6-4 μJ, steady pulse width of 38-40 ns and peak power of 165-105 W were achieved at repetition rate of 0.1-1.5 kHz. Efficient Q-switching at higher repetition rates is expected when a more intense pump source is employed.